1. Field of the Invention
The present invention relates to detectors based on intraband transitions in quantum well structures and more specifically to an electrically-tunable infrared detector exhibiting large responsivity, sharp line width, which determines resolution of the detector, and wide tunability.
2. Prior Art
Infrared detectors in the prior art have various advantages, both in terms of quality measures such as responsivity, bandwidth, speed, and tunability, and in terms of such things as ease of fabrication, reliability, and cost. All related detectors rely on specific quantum transitions to convert electromagnetic radiation to a detectable voltage or current. The most commonly used such transitions can be divided into three types: interband transitions, confined-to-confined intraband transitions, and confined-to extended intraband transitions.
In interband transitions, an electron is excited from a state in the valance band (either from the bulk valence band, or from a heterostructure confined state) to states in the conduction band. The absorption for this kind of transition can be very strong, due to the large number of available electrons in the initial state.
In confined-to-confined intraband transitions, both the initial and final states are confined conduction-band or valence-band states induced by band-gap engineering. In this case, the absorption is due to envelope function overlap, and this transition probability can be very large. In many cases this results in absorption features as strong for a single quantum well of 100 .ANG. as for bulk samples of thickness of a few 1000 .ANG. (the order of an absorption length). While these transitions have the advantage of a more sharply defined absorption bandwidth, they often suffer in terms of population of the initial state.
Finally, there are confined-to-extended intraband transitions, in which the transition is between an initial confined-state and the conduction-band continuum. It was found that designing the structure in such a way that the quasi-confined final state of the device is pushed up into the continuum allows for a stronger transition probability while still maintaining a fairly well-defined absorption bandwidth.
In intraband transitions, both confined-to-confined and confined-to-extended, selection rules require that the electric-field vector of the detectable light be parallel to the direction of growth. This means that for unpolarized light at least 50% is lost to this constraint. It also means that for maximum detection, the light must enter the detector edge-on. This places constraints on the configuration of the detector.
In all absorption processes, the population of the initial state and the number of final states available are as important as the transition probability. It is the product of these factors that determine the total absorption and hence the efficiency of the detector.
The best known of infrared devices are the narrow band-gap alloy detectors that depend on the interband band-gap transition of the semiconductor. While providing the best figures of merit to date for commercially produced detectors, after many years of development, the growth and processing of these materials continue to be a challenge. Effort still continues in this field, including attempts to grow the alloy using molecular beam epitaxy (MBE) to improve reliability and uniformity, and to grow bandgap-engineered structures. However, such structures do not exhibit electrical tunability.
In electrically tunable infrared detectors the wavelength tunability achievable in Quantum Well Infrared Photodetectors (QWIPs) due to an applied electric field (Stark shift) is quite limited. To first order, the energy levels in a single quantum well all move equally with applied field, and thus any change in the energy spacing between levels is second order at best. To achieve appreciable tunability requires extensive modification of the well shape.
While narrow band-gap alloys continue to be the most commonly used technology, concerns over the growth and fabrication difficulties involved with these materials have caused researchers to explore alternate methods of detecting infrared light. First was the use of quantum wells as sources of confined electrons, taking advantage of the increase in transition probability with little regard for the niceties of the initial state energies. Then came a detector which used selection of a particular energy state in a single quantum well, transitioning to a quasi-continuum final state, and later to an even more strongly confined superlattice band final state, to give good frequency selection. This approach suffered from a low initial state population, and thus had relatively low quantum efficiencies.
The difficulties of low initial state population and the associated low quantum efficiency were solved by using a multi-period structure and using a waveguide to allow multiple passes through the device, bringing the quantum efficiency to 95% for polarized light. Large absorption was achieved by coupling to a quasi-continuum state and the dark current was lowered by widening the barrier. This resulted in a device that, by some measures, is competitive with state-of-the-art direct bandgap material systems.
Many of the prior art intraband detectors share certain characteristics. First, they take advantage of quantum states formed by interfaces or actual wells created by growth or doping. These effects can all be expressed as a change in the energy potential seen by the detection particle. Next, they all depend on a photon induced transition of the detection carrier from a well confined state to a less confined one. The efficiency of the detector is directly dependent upon the strength of this transition and thus, in an ideal detector, transition strength should be as large as possible. Because the dark current depends on the confinement of the initial state, and the escape rate of the final state affects the number of carriers that finally reach the contact, it is desirable to maximize the first, while minimizing the second.
Most of the prior art intraband detectors also use an electric field to sweep the detection carrier to the contact and, in the case of electrically-tunable devices, use the electric field to change the wavelength of the detected photon through Stark shift.
Finally, all of the prior art intraband detectors are based on simple conceptual designs: well to well, first to second state in a simple well shape, etc.
It is desirable to provide more efficient quantum structures that simultaneously maximize those quantities that are of the most importance for a given application. There is tremendous freedom in the selection of growth parameters in a band-gap engineered material, which is largely unexplored.
It is particularly desirable to provide a quantum well electrically-tunable infrared detector having a superior structure, especially wide tuning range and good signal-to-noise.